Electric heater



Jan. 8, 1957 J. P. DURAY ET AL I 2,777,043

ELECTRIC HEATER Filed July 6, 1954 Inventor JOSEPH P. DUIZAY WILL/Al"!E. K022 United States Patent 2,777,043 ELECTRIC HEATER Joseph P. Durayand William E. Kurz, Chicago, 111., as-

signors to Precision Scientific Company, Chicago, 1th, a corporation ofIllinois Application July 6, 1954, Serial No. 441,604

Claims. (Cl. 219-19) This invention relates to electric heaters and hasparticular reference to coiled wire heater assemblies employed as smallstoves.

Electric stoves or plates of the type employing an inexpensive heaterelement in the form of a coil of an electrical resistance wire have beencharacterized by relatively large thermal inertia. The slow rate ofchange of the heater or stove temperature works to a disadvantage inmany instances. Thus in installations Where a hydrocarbon or otherliquid may be heated in distillation equipment for fractionating, it isimportant that the heater have a fast thermal response to change in theelectrical input energy in order that the liquid boiling point can bereadily controlled. However, despite the ease with which electricalinput energy itself can be controlled, the requirements for supportingand spacing the heater coil in order that the heating capacity over agiven area can be made suificiently large has also resulted in heating alarge mass of insulating material. The time of thermal response, as wellas the efficiency of the unit in terms of electrical energy converted tousefully directed heat, are adversely affected.

It is therefore an object of the invention to provide an improvedelectric heater or stove having low thermal inertia.

It is also an object of the invention to provide a more efiicient stoveor heater, advantageously providing both radiant heat energy and theconvection of heated air.

It is another object of the invention to provide an inexpensive smallelectric stove of the type having a coiled Wire heater unit which isboth inexpensive in construction and relatively free of maintenancerequirements.

It is still another object of the invention to provide a coiled wireheating unit having an improved support means facilitating low thermalinertia and high effective efliciency.

it is a further object of the invention to provide in a coiled wireelectrical heater an improved insulating support means which minimizesthe danger of damage or shorting of the heater coil due to eitherextended use, abuse, or spilling of materials on the heater.

It is a still further object of the invention to provide an inexpensivesmall stove capable of long life and having easily maintained orreplaced parts.

The objects of the invention thus generally set forth together withother objects and ancillary advantages are attained by the constructionand arrangement shown by Way of illustration in the accompanyingdrawings in which:

Figure 1 is a top view, partly in section, of an electric stove unitincorporating the invention together with a schematic representation ofa controlled electrical energy source therefor.

Fig. 2 is 'a sectional view along line 22 of Fig. 1 and also showing indotted outline the position of a flask and flask holder on the stove.

Fig. 3 is an end view partly in section of the stove of Fig. l; and

Patented Jan. 8, 1957 Fig. 4 is a perspective view of one of theinsulating spacers suitably employed in the stove of Fig. 1.

While the invention is susceptible of various modifications andalternative constructions, there is shown in the drawing and will hereinbe described in detail as a preferred embodiment a flask heater having asmall capacity, being rated in the order of a few hundred watts, but itis to be understood that it is not thereby intended to limit theinvention to the form disclosed, but it is intended to cover allmodifications, alternative constructions, and equivalents falling withinthe spirit and scope of the invention as expressed in the appendedclaims.

Referring to the drawings, the heater shown includes a top plate orpanel 1 of asbestos board or other suitable material preferably havingsmall thermal storage capacity and conductivity so that the panel itselfwill not be heated unduly. This panel may be suitably supported in ahorizontal position by legs (not shown) either as a single unit or incombination With other heaters. A circular opening 2 in the panel isdesigned to expose the bottom of a fiask 3 which is exposed to radiationand heated air from the heater unit below the panel. As particularlyindicated in Fig. 2, the flask 3 may have a curved bottom surface, inwhich case it is suitably supported by an auxiliary support pad 4 whichrests over the aperture 2 in the panel 1 and is itself apertured tosupport a particular size of flask and expose its bottom surface to theheater. Obviously many-types of stove tops and holder arrangements maybe employed for supporting the object to be heated on or above theheater unit without departing from the spirit of the invention.Likewise, the heater unit may be variously supported or enclosed as maybe desired for the appearance, thermo-insulation, or other requirementsof particular types of installations.

In accordance with the invention, the heater unit itself comprises agenerally U-shaped support base or channel 5 which holds a plurality ofspaced insulating bars 6 which in turn between them continuously supportlengths of a heating coil 7. As may be seen from the drawings, the basemember 5 suitably takes the form of a rectangular metal sheet havingvertical side members 8 bent up from the substantially flat bottomsurface 9. Support flanges 10 are suitably bent over from the upperportions of the side walls 8 and form a convenient means for attachingthe base to the underside of the stove top panel 1, flush flat-headmachine screws 11 being suitably used for that purpose. A plurality ofvertically elongated slots 12 are punched or otherwise formed in ahorizontal array in each side Wall 8 of the base, the slots beinguniformly spaced and oppositely alined in the facing side walls toaccommodate the ends of the spacer bars '6.

The base or channel member 5 also additionally serves to increase theheater effectiveness. Toward this end a horizontally elongated slot 13running almost the entire width of the base member is provided in thelower portion of each side wall 8 below the hold-ing slots 12. Theseslots 13 together with the passages provided by the pair of open sidesof the channel 5 help give rise to the convection air currents, the airentering under the heater element rising through it to the flask 3above. The bottom surface 9 of the base is not itself apertured orperforated since it serves as a reflecting shield for radiant energyfrom the heater element. Thus a portion of any downwardly radiated heatenergy is reflected upwardly again toward the flask 3. For this purpose,the base member is preferably made of polished aluminum, or, where theheater has a higher temperature rating as may be required for particularinstallations, of polished stainless steel. As shown in the drawings,the bottom surface 9 of the base member 5 may also be formed with acircular boss or depression 14, principally for the purpose ofincreasing the rigidity of 3 that member, although such a feature may beomitted if desired;

The spacer bars 6 are designed to be locked or anchored in placed in an'open parallel array between. the side walls 8 of the base member 5. Asshown in Fig. 4, each of the bars is made of a refractory insulatingmaterial which may be readily molded to the desired form. Porcelain orother ceramic material is suitable, such materials having bothelectrical and thermal insulating properties. Each of the spacer barshas a basic rectangular form with a reduced thickness portion or rib 15defined between its end portions and above a wider bottom surface orledge 16. A smooth curved transition or fillet is preferably providedfrom the ledge to the rib to accommodate the lower side portions of theheater coil and to prevent accumulation of dirt or spilled materials.The effect is thus generally to provide a rib having a bottom portionwhich is wider than the top portion so that when the ribs are arrangedin uniformly spaced parallel array, their top portions or ribs are morewidely spaced than the bottom portions. At the end of each narrowed ribportion 15 is a notch 17 extending downwardly from the top surface andinwardly for the purpose of accommodating a heater wire and holding itin place. Beyond each full diameter end portion of each spacer bar is areduced diameter noncircular end projection 18, this projection takingthe form in Fig. 4 of a vertically elongated oval cross sectiondimensioned to fit within the slot 12 in a base side wall 8.

The spacer bars are inserted in position in the base with each endportion 18 extending through one of the slots 12 before the base memberis bolted in place under the top panel 1', the sheet metal channel basebeing easily bent or sprung outwardly to admit the spacer bars. A smallamotmt of end play for the spacer bars is preferably provided, thedistance between the full diameter end portions of each bar beingsomewhat less than the distance between the base side walls, thusallowing the bars to expand and contract with changes in temperature andto prevent binding or stresses due to variations of dimension withinmanufacturing tolerances. At the same time, however, the bars are lockedor anchored in place once the base has been fastened in place and heldagainst rotation. The number of spacer bars and their length dependsupon the desired size of the heater area, since it is the space betweenadjacent bars that provides or defines a position for each length of theheater coil 7.

The heater coil 7 itself is suitably made of conventional heater wiresuch as a nickel-chrornium alloy helically wound with spaced turns. Suchheater elements conveniently provide a relatively large heat dissipationrating and a practical resistance range for commonly availableelectrical power supplies. Successive straight lengths of the heatercoil are defined between stretched turns 19 and positioned between eachadjacent pair of facing spacer sides. The coil is anchored in a zig-zagor folded form, the wire of the stretched or pulled apart turns 19 beingfitted within a slot 16 of the spacer bar between adjacent coilsections. The coil assembly is held firmly in place since the notches 17are undercut. While for this simple self-connected series of coillengths only alternate spacer notches 16 are employed, the spacer bars 6are in this case shown with notches 16 at each end since it is generallymore economical to provide both notches than to require end-for-endalternation of single-notch bars in assembly. The ledge 16 formed by thebroadened base portion of the spacer bar prevents the coil from droppingdown through the open space between the base surfaces of adjacent barswhich is slightly less than the coil diameter but still providescontinuous support along each coil length. Efiective retention of thecoil in proper position is thus assured since in an open support framethe coil wouldotherwise tend to sag or droop due to its annealing withuse.

Suitable terminals 20 are provided for the ends of the coil, a terminalboard 21, also suitably made of asbestos board, being convenientlyprovided for this purpose and attached to the underside of the top panel1 through the same screws that hold one of the base flanges in place.

In operation, since the heater assembly described has a quick thermalresponse, it may be advantageously employed with graduated or continuouscurrent control means. Thus, as shown in Fig. 1, the heater element isenergized from a convenient electrical energy source 22 such as theusually available ll5-volt, 60'-cycle supply through a variabletransformer 23, although of course other means of varying the currentsuch as an adjustable resistor may beemployed. A wattmeter 24 may alsobe additionally connected in circuit so that an instantaneous reading ofthe energy supplied to the heater element is available as an accuratetemperature reference. is very advantageously employed in heating ahydrocarbon liquid in the flask 3 for distillation tests, the boilingpoint of the liquid may be readily controlled by varying the transformerratio and thus controlling the distillation of the various fractions.

The mass of the heater assembly which is subject to increase or decreaseof temperature along with the change in temperature of the heater wireitself is minimized, but the space between the heater elements minimizesconductive heating of the stove or heater assembly itself. Instead, aportion of the radiant energy of the coil which is directed downwardlyfrom the bottom side of the heater coil isreflected upwardly uponincidence with the polished reflective bottom surface 9 of the heaterbase so that it may be directed upwardly between the spacer bars todirectly heat the flask; Not all of the heating is by radiation, ofcourse, and again the spacing between the heater bars facilitates theflow of convection air currents, the heated air rising to also heat theflask. Thus it may be seen that not only is the thermal response timemade very fast, but the thermal efficiency of the unit so far as itsapplication of the heat generated to the flask is also increased. Inthis way, the wattmeter reading is made more significant as to accuracyas well as to time, further facilitating manual control of the heaterfor particular control or test requirements.

At the same time, the structure of the unit itself is seen to be sosimple as to permit it to be inexpensively manufactured and maintained.The heater coils are assured of long life by their protection againstshorting through adequate and continuous support so that the annealingof the heater with age does not result in sagging and dislocation of theheater. Accidental spilling of the liquid being heated causes minimumdamage to the coil in so far as the liquid is free to pass through thespaces between the insulating bars. Replacement of the heater coils whennecessary is immediately effected by reason of the fact that the baseassembly is easily removed and sprung or bent to permit replacement ofthe individual spacers.

We claim as our invention:

1. A quick response electric heater comprising a channel-shaped supportbase, each channel side being provided with a horizontal array of spacedapertures, a plurality of separate insulating spacer bars positionedwithin said channel with their opposite ends anchored in said channelside apertures, said spacer bars being formed to provide a lesserspacing between their bottom portions than between their top portions,and a coiled wire heater element having lengths thereof positionedbetween adjacent pairs of said spacer element sides, said element havinga coil diameter greater than said lesser spacing whereby said heatercoil lengths are supported between said spacer bars.

2. A quick response electric heater comprising a channel-shaped sheetmetal support base having a heat reflect- With the particular apparatusshown, which ing inner bottom surface, each channel side being providedwith a horizontal array of uniformly spaced apertures, a plurality ofseparate spaced insulating bars positioned Within said channel withtheir opposite ends supported in said channel side apertures, saidspacer bars being formed to provide a lesser spacing between theirbottom portions than between their top portions, and a coiled wireheater element having a coil diameter greater than said lesser spacingwith successive self-connected lengths thereof positioned betweenadjacent pairs of said spacer element sides whereby said heater coillengths are supported between said spacer bars, each of said spacer barsbeing notched near an end thereof to retain a connecting wire betweenadjacent heater coil lengths.

3. A quick response electric stove comprising a top panel having anaperture therein, a channel-shaped sheet metal support base under saidpanel with the inner bottom surface of the channel disposed under saidaperture, said surface being highly reflective of radiant heat energy,each channel side being provided with a horizontal array of uniformlyspaced apertures, a plurality of separate insulating spacer barspositioned within said channel with their opposite ends supported insaid channel side apertures, said spacer bars being formed to provide aspacing between their bottom portions smaller than between their topportions, and a coiled wire heater element having a coil diametergreater than said smaller spacing with successive self-connected lengthsthereof positioned between adjacent pairs of said spacer element sides,each of said spacer bars being notched near an end thereof to retain aconnecting wire between adjacent heater coil lengths.

4. A quick response electric heater comprising a top panel having anaperture therein, a channel-shaped polished sheet metal support basefastened to the under side thereof with the inner bottom surface of thechannel disposed under said aperture for reflecting heat energy thereto,each channel side being provided with a horizontal array of uniformlyspaced apertures, a plurality of separate uniformly spaced refractoryinsulating bars positioned within said channel with their opposite endssupported in said channel side apertures, each of said spacer barshaving a lower edge on each side along its intermediate length, and acoiled wire heater element having successive self-connected lengthsthereof retained between adjacent pairs of said spacer element sides onsaid ledges, each of said spacer bars being transversely notched nearone end thereof to anchor a connecting wire between adjacent heater coillengths.

5. A quick response flask heater comprising a top panel having anaperture therein, a channel-shaped polished sheet metal support basefastened to the under side thereof with the inner bottom surface of thechannel disposed under said aperture for reflecting heat energy thereto,each channel side being provided with a horizontal row of uniformlyspaced apertures, a plurality of separate uniformly spaced refractoryinsulating bars spaced within said channel above said bottom surfacewith their opposite ends supported in said channel side apertures, eachof said spacer bars having a lower ledge on each side along itsintermediate length and being transversely notched near one end, and acoiled electric resistance wire heater element having successiveself-connected lengths thereof retained between adjacent pairs of saidspacer element sides on said ledges with connecting wires betweenadjacent heater coil lengths secured in said notches, said channel sidebeing additionally apertured below said row of apertures.

References Cited in the file of this patent UNITED STATES PATENTS1,057,745 Kohn Apr. 1, 1913 1,094,264 Steward Apr. 21, 1914 1,682,402Mu'lvany Aug. 28, 1928 1,705,717 Colby Mar. 19, 1929 1,855,507 BathrickApr. 26, 1932 2,265,549 Shaw Dec. 9, 1941

